Wavelength control of two-channel DEMUX/MUX in silicon photonics

ABSTRACT

Method and devices of controlling wavelengths in two-channel DEMUX/MUX in silicon photonics are provided. The two-channel DEMUX/MUX includes a waveguide-based delay-line-interferometer at least in receiver portion of a two-channel transceiver for DWDM optical transmission loop and is configured to split a light wave with combined two-wavelengths into one light wave with locked one channel wavelength and another light wave with locked another channel wavelength. The waveguide-based delayed-line interferometer (DLI) is characterized by a free-spectral-range configured to be equal to twice of channel spacing. The method includes tuning heater of DLI in receiver of each two-channel transceiver by using either low-frequency dither signals added on MZMs associated with respective two channels as feedback signal or one DFB laser wavelength tapped from an input of transmitter portion at one channel before or after the MZMs as a direct wavelength reference to feed into an output of receiver portion at another channel.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/536,294 filed Nov. 7, 2014, the entire content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides DEMUX/MUXconfigurations of 2-channel silicon photonic devices for wavelengthcontrol and methods thereof.

Over the last few decades, the use of communication networks exploded.In the early days of Internet, popular applications were limited toemails, bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Progress in computer technology (and the continuation of Moore's Law) isbecoming increasingly dependent on faster data transfer between andwithin microchips. Optical interconnects may provide a way forward, andsilicon photonics may prove particularly useful, once integrated on thestandard silicon chips. 40-Gbit/s and then 100-Gbit/s data rates WDMoptical transmission over existing single-mode fiber is a target for thenext generation of fiber-optic communication networks. The big hangup sofar has been the fiber impairments like chromatic dispersion that areslowing the communication signal down. Everything is okay up to 10Gbits/s plus a little, but beyond that, distortion and attenuation taketheir toll. Many approaches are proposed on modulation methods fortransmitting two or more bits per symbol so that higher communicationrates can be achieved. Mach-Zehnder modulators (MZM) can handle thehigher data rates but require a driver that is differential with a wideoutput voltage swing. Beyond the light modulation for data transmission,the MUX/DEMUX of light signals is an essential building block for theoptical network based on silicon photonics.

Silicon photonic devices can be made using existing semiconductorfabrication techniques, and because silicon is already used as thesubstrate for most integrated circuits, it is possible to create hybriddevices in which the optical and electronic components are integratedonto a single microchip. In particular, silicon photonic devices havebeen applied in dense-wavelength-division multiplexing (DWDM) opticaltransmission networks, in which DEMUX/MUX of light signals requireprecise wavelength targeting and control over environment temperaturechange. Therefore, improved wavelength control techniques and methodsare desired.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a two-channelDUMUX/MUX device for waveguide-based wavelength control in siliconphotonics. Merely by examples, the present invention discloses severalDEMUX/MUX configurations of 2-channel silicon photonic devices foroptical transmission and reception with wavelength control functionintegrated in a single waveguide chip for high data rate WDM opticalcommunications, though other applications are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

In an embodiment, the present invention provides a two-channel siliconphotonic transceiver with wavelength control for DEMUX/MUX in DWDMapplications. The two-channel silicon photonic transceiver includes atransmitter comprising a first optical input port and a second opticalinput port. The transmitter further includes a first modulator and asecond modulator respectively coupled to the first optical input portand the second optical input port for modulating a first optical signalat a first wavelength and a second optical signal at a secondwavelength. Additionally, the transmitter includes a 2×1 WDM combinercoupled to the first modulator and the second modulator to transmit afirst transmission signal combined with the first wavelength and thesecond wavelength to a first optical output port. Additionally, thetwo-channel transceiver includes a receiver comprising a third opticalinput port, a second optical output port, a third optical output port,and a two-channel delay-line interferometer coupled between the thirdoptical input port and second and third optical output ports. The thirdoptical input port is configured to receive a second transmission signalcombined with the first wavelength and the second wavelength. The secondtransmission signal is substantially similar to the first transmissionsignal. The delayed-line interferometer in the receiver is configured tosplit the second transmission signal to a third optical signal locked atthe first wavelength to the second optical output port substantiallyfree of element of the second wavelength and a fourth optical signallocked at the second wavelength to the third optical output portsubstantially free of element of the first wavelength.

In an alternative embodiment, the present invention provides a method oflocking channel wavelengths through two-channel DEMUX/MUX in DWDMapplications. The method includes disposing a first two-channeltransceiver at a first terminal of a DWDM communication loop and asecond two-channel transceiver at a second terminal of the DWDMcommunication loop. Each of the first and the second two-channeltransceiver includes a transmitter having a first/second optical inputport coupled with a first/second modulator and a 2×1 WDM combinercoupled to a first optical output port and a receiver comprising a thirdoptical input port connected to a two-channel delay-line interferometerincluding a heater with a second/third optical output port. The methodfurther includes coupling a first/second DFB laser light wave to thefirst/second optical input port of the transmitter of the firsttwo-channel transceiver. Additionally, the method includes modulatingthe first/second DFB laser light wave with a first/second wavelength atthe first/second modulator to generate a first/second optical signalcharacterized by the first/second wavelength and combining the firstoptical signal with the second optical signal to a first transmissionsignal outputted through the first optical output port to a firstoptical fiber. The method further includes inserting a first/seconddither signal to the first/second modulator. The first/second dithersignal is carried by the first transmission signal. Furthermore, themethod includes receiving the first transmission signal including thefirst/second dither signal from the first optical fiber by the thirdoptical input port of the receiver of the second two-channeltransceiver. The method further includes setting the two-channeldelay-line interferometer in the receiver with a free-spectral rangeequal to twice of channel spacing between the first wavelength and thesecond wavelength for splitting the first transmission signal into athird optical signal including the first dither signal into the secondoptical output port and a fourth optical signal including the seconddither signal into the third optical output port. Moreover, the methodincludes detecting the first/second ditcher signal at the second/thirdoutput port of the receiver of the second two-channel transceiver by aphotodiode to obtain a first/second electrical signal and tuning theheater of the two-channel delay-line interferometer via a feedbackcircuit for maximizing the first/second electrical signal, therebycontrolling the third/fourth optical signal at the second/third opticaloutput port to be substantially free from element of the second/firstwavelength.

In yet another alternative embodiment, the present invention provides amethod of locking channel wavelengths through two-channel DEMUX/MUX inDWDM applications. The method includes disposing a first two-channeltransceiver at a first terminal of a DWDM communication loop and asecond two-channel transceiver at a second terminal of the DWDMcommunication loop. Each of the first and the second two-channeltransceiver includes a transmitter having a first/second optical inputport coupled with a first/second modulator and a 2×1 WDM combinercoupled to a first optical output port and a receiver comprising a thirdoptical input port connected to a two-channel delay-line interferometerincluding a heater with a second/third optical output port. The methodfurther includes coupling a first/second DFB laser light wave to thefirst/second optical input port of the transmitter of the firsttwo-channel transceiver and tapping 2-10% power of the second/first DFBlaser light wave then fed to the second/third optical output port of thereceiver of the first two-channel transceiver as a reference signal.Additionally, the method includes modulating the first/second DFB laserlight wave with a first/second wavelength at the first/second modulatorto generate a first/second optical signal characterized by thefirst/second wavelength and combining the first optical signal with thesecond optical signal to a first transmission signal outputted throughthe first optical output port to a first optical fiber. The methodfurther includes receiving the first transmission signal by the secondtwo-channel transceiver and sending a second transmission signal to asecond optical fiber, the second transmission signal being characterizedby the first wavelength and the second wavelength. The method alsoincludes receiving the second transmission signal from the secondoptical fiber by the receiver of the first two-channel transceiver andsetting the two-channel delay-line interferometer in the receiver with afree-spectral range equal to twice of channel spacing between the firstwavelength and the second wavelength for splitting the secondtransmission signal into a third optical signal into the second opticaloutput port and a fourth optical signal into the third optical outputport. Furthermore, the method includes detecting the reference signal bya photodiode next to the third optical input port of the receiver of thefirst two-channel transceiver to obtain an electrical signal. Moreover,the method includes tuning the heater of the two-channel delay-lineinterferometer via a feedback circuit for maximizing the electricalsignal, thereby controlling the third/fourth optical signal at thesecond/third optical output port to be substantially free from elementof the second/first wavelength.

In still another alternative embodiment, the present invention providesa method of locking channel wavelengths through two-channel DEMUX/MUX inDWDM applications. The method includes disposing a first two-channeltransceiver at a first terminal of a DWDM communication loop and asecond two-channel transceiver at a second terminal of the DWDMcommunication loop. Each of the first and the second two-channeltransceiver includes a transmitter having a first/second optical inputport coupled with a first/second modulator and a 2×1 WDM combinercoupled to a first optical output port and a receiver comprising a thirdoptical input port connected to a two-channel delay-line interferometerincluding a heater with a second/third optical output port. The methodfurther includes coupling a first/second DFB laser light wave to thefirst/second optical input port of the transmitter of the firsttwo-channel transceiver. Additionally, the method includes modulatingthe first/second DFB laser light wave with a first/second wavelength atthe first/second modulator to generate a first/second optical signalcharacterized by the first/second wavelength and tapping 2-10% power ofthe second/first optical signal after the second/first modulator, thenfeeding the power to the second/third optical output port of thereceiver of the first two-channel transceiver as a reference signal. Themethod further includes combining the first optical signal with thesecond optical signal to a first transmission signal outputted throughthe first optical output port to a first optical fiber. Furthermore, themethod includes receiving the first transmission signal by the secondtwo-channel transceiver and sending a second transmission signal to asecond optical fiber, the second transmission signal being characterizedby the first wavelength and the second wavelength. The method alsoincludes receiving the second transmission signal from the secondoptical fiber by the receiver of the first two-channel transceiver andsetting the two-channel delay-line interferometer in the receiver with afree-spectral range equal to twice of channel spacing between the firstwavelength and the second wavelength for splitting the secondtransmission signal into a third optical signal into the second opticaloutput port and a fourth optical signal into the third optical outputport. The method further includes detecting the reference signal by aphotodiode next to the third optical input port of the receiver of thefirst two-channel transceiver to obtain an electrical signal. Moreover,the method includes tuning the heater of the two-channel delay-lineinterferometer via a feedback circuit for maximizing the electricalsignal, thereby controlling the third/fourth optical signal at thesecond/third optical output port to be substantially free from elementof the second/first wavelength.

The present invention achieves these benefits and others in the contextof known waveguide laser communication technology. However, a furtherunderstanding of the nature and advantages of the present invention maybe realized by reference to the latter portions of the specification andattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1A is a simplified diagram of a two-channel transceiver including a3 dB combiner as MUX and a waveguide-based delay-line-interferometer asDEMUX according to an embodiment of the present invention.

FIG. 1B is a simplified diagram of a two-channel transceiver including awaveguide-based delay-line-interferometer with free-spectral-range equalto twice of channel spacing as either DUMUX or MUX according to anembodiment of the present invention.

FIG. 2 is a simplified diagram of a two-channel waveguide-baseddelay-line-interferometer with free-spectral-range equal to twice ofchannel spacing according to an embodiment of the present invention.

FIG. 3 is a simplified diagram of a pair of two-channel transceiversusing tones for wavelength controlled transmission according to anembodiment of the present invention.

FIG. 4 is a simplified diagram of a pair of two-channel transceiversusing tapped DFB signal for wavelength controlled transmission accordingto another embodiment of the present invention.

FIG. 5 is a simplified diagram of a pair of two-channel transceiversusing tapped DFB signal after MZM including tones for wavelengthcontrolled transmission according to yet another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a two-channelDUMUX/MUX device for waveguide-based wavelength control in siliconphotonics. Merely by examples, the present invention discloses severalDEMUX/MUX configurations of 2-channel silicon photonic devices foroptical transmission and reception with wavelength control functionintegrated in a single waveguide chip for high data rate WDM opticalcommunications, though other applications are possible.

In the last decades, with advent of cloud computing and data center, theneeds for network servers have evolved. For example, the three-levelconfiguration that have been used for a long time is no longer adequateor suitable, as distributed applications require flatter networkarchitectures, where server virtualization that allows servers tooperate in parallel. For example, multiple servers can be used togetherto perform a requested task. For multiple servers to work in parallel,it is often imperative for them to be share large amount of informationamong themselves quickly, as opposed to having data going back forththrough multiple layers of network architecture (e.g., network switches,etc.).

Leaf-spine type of network architecture is provided to better allowservers to work in parallel and move data quickly among servers,offering high bandwidth and low latencies. Typically, a leaf-spinenetwork architecture uses a top-of-rack switch that can directly accessinto server nodes and links back to a set of non-blocking spine switchesthat have enough bandwidth to allow for clusters of servers to be linkedto one another and share large amount of data.

In a typical leaf-spine network today, gigabits of data are shared amongservers. In certain network architectures, network servers on the samelevel have certain peer links for data sharing. Unfortunately, thebandwidth for this type of set up is often inadequate. It is to beappreciated that embodiments of the present invention utilizes PAM(e.g., PAM4, PAM8, PAM12, PAM16, etc.) in leaf-spine architecture thatallows large amount (up terabytes of data at the spine level) of data tobe transferred via optical network.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 1A is a simplified diagram of a two-channel transceiver including a3 dB combiner as MUX and a waveguide-based delay-line-interferometer asDEMUX according to an embodiment of the present invention. This diagramis merely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown, the two-channelWDM transceiver 100 includes a two-channel transmitter having twooptical inputs, 101 and 102, each receiving a laser light from acorresponding DFB laser source with a predetermined wavelength, λ1 orλ2. Each laser light is modulated by a MZM modulator, 112 or 113, togenerate an optical signal bearing the corresponding wavelength, λ1 orλ2. The two-channel transmitter further includes a 3 dB power combiner110 as a MUX device to combine the two optical signals into one opticaloutput 115. The MUX 110 can be either a MMI combiner or DC combiner.

Additionally, the two-channel WDM transceiver 100 includes a two-channelreceiver having an optical input 125 configured to receive one or morewavelength signals into a waveguide-based delay-line-interferometer(DLI) 120. For example, a two-wavelength (λ1 and λ2) combined signal isreceived at the optical input 125 of an 1×2 splitter before entering theDLI 120. The DLI 120 is configured to have one path chosen to be longerthan another path by a specific length so that its free spectral rangeis equal to twice of channel spacing of the two wavelengths (λ1 and λ2).The longer path also includes a resistive heater for wavelength controladjustment. Therefore, the DLI 120 acts as a deinterleaver or a DEMUXdevice which is able to split the signal from the input 125 withcombined two wavelengths into two signals respectively output to twooptical outputs, 121 and 122, each carrying a signal with a singlewavelength. For example, output 121 carries λ1 at channel 1 (CH1) andoutput 122 carries λ2 at channel 2 (CH2), each being independentlydetected by a photodiode.

In an embodiment, a photodiode can also be coupled next to the opticalinput 125 of the DLI 120 for measuring any reflected signal. The DLI 120in the receiver needs to be tuned so that light with wavelength λ1 goesto CH1 PD and light with wavelength λ2 goes to CH2 PD. This can be doneby using the DFB laser light wavelength as a wavelength reference. In aspecific embodiment, a small portion (typically 2˜10%) of CH2 DFB (orCH1 DFB can be used alternatively) is tapped and fed into the receiverDLI 120 via a reverse CH1 path from output 121 (or CH2 path from output122 can be used alternatively). Then the heater on the DLI 120 is tunedso the power at PD coupled next to the optical input 125 is maximized.In such a way, the DLI 120 is tuned to provide a free spectral rangeequal to the channel spacing between λ1 and λ2 so that the resultedinterference spectral gives desired interleaving wavelengthsrespectively into CH1 output 121 and CH2 output 122. More descriptionabout the DLI tuning will be found in subsequent sections of thespecification.

FIG. 1B is a simplified diagram of a two-channel transceiver including awaveguide-based delay-line-interferometer with free-spectral-range equalto twice of channel spacing as either DUMUX or MUX according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. As shown, the two-channel WDM transceiver 100B issubstantially the same as the two-channel WDM transceiver 100 exceptthat the MMI combiner 110 in the transmitter is replaced by awaveguide-based DLI 120A while in the receiver a substantially the samewaveguide-based DLI 120B is retained therein. Other configurations andwavelength control schemes are the same as those described above. Eachof the two DLIs, 120A and 120B, contains a resistive heater configuredfor wavelength control by adjusting interference spectrum so that thefree spectral range is equal to the channel spacing between λ1 and λ2.

FIG. 2 is a simplified diagram of a two-channel waveguide-baseddelay-line-interferometer with free-spectral-range equal to twice ofchannel spacing according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. As shown, thetwo-channel waveguide-based delay-line-interferometer 200 includes atleast one input 201 receiving a light wave with two wavelengths λ1 andλ2, a 1×2 power splitter 224 to split the light wave received from theinput 201 to two paths 221 and 222, a 2×2 coupler 225 coupled betweenthe two paths 221, 222 and two outputs 231, 232. The two paths 221 and222 are silicon-based waveguide built on a single-chip SOI substrate.Particularly, one waveguide path, e.g., 221, is designed to be longerthan another waveguide path 222 to give a delayed phase for path 221.Each path, either 221 or 222, still carries light signal with bothwavelengths λ1 and λ2. When the two paths are merged at the 2×2 coupler225 the light signal with delayed phase in path 221 and wavelength λ1(or λ2) interferes with the light signal in path 222 with the samewavelength λ1 (or λ2) to generate two transmission interferencespectrums separated due to wavelength difference between λ1 and λ2 aswell as delayed phase caused by length difference between the two paths221 and 222.

In a specific embodiment, the two-channel waveguide-baseddelay-line-interferometer (DLI) 200 includes a resistive heater 210disposed to one, usually the longer one, path 221, to provide a tuningmechanism for the delayed phase by changing temperature to causerefractive index change through the corresponding waveguide. In anembodiment, for each output 231 or 232 the DLI 200 acts as a filterallowing a light wave with only one wavelength (either λ1 or λ2) topartially pass out. In addition to the extra length in path 221, thetemperature change provided by the heater 210 is able to further adddelayed phase for the path 221 relative to the path 222. As a result,the output transmission interference spectrum (ouput1) of the light wavewith two wavelengths λ1 and λ2 at output 231 versus the outputtransmission interference spectrum (ouput2) at output 232 will move leftor right along the wavelength axis depending on the heater power.

Each of the transmission interference spectrums produced by the DLI 200is characterized by a plurality of passbands respectively distributedaround λ1 (or λ2) at a first harmonic peak and periodically around aplurality of higher harmonic peaks. The peak-peak spacing is defined asa free-spectral-range (FSR). For the light wave with two wavelengths λ1and λ2 in the present case, in a specific embodiment, FSR is tuned to beequal to twice of the difference between λ1 and λ2. In such a manner,the DLI 200 filter acts exactly as a deinterleaver that split the lightwave with two wavelengths into two separate light waves each with onewavelength.

FIG. 3 is a simplified diagram of a pair of two-channel transceiversusing tones for wavelength controlled transmission according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. As shown, the pair of two-channel transceivers(Transceiver 1 and Transceiver 2) is respectively disposed attransmitting and receiving ends as MUX and DEMUX of a two-way trafficoptical communication loop including wavelength control. For example inthe transmission path of the optical communication loop, Transceiver 1is configured to use two DFB laser sources with light wavelength at λ1and λ2 being respectively modulated on two MZMs to form two opticalsignals before being combined into one output for transmitting throughoptical fiber. In an embodiment, the DFB laser wavelengths λ1 and λ2 areselected to be two neighboring channels of ITU grid with a channelspacing of 100 GHz, or 50 GHz, or 25 GHz, or others. The Transceiver 1can be the two-channel transceiver 100 described in FIG. 1A.Alternatively, the Transceiver 1 can be the two-channel transceiver 100Bwith the waveguide-based DLI as a 2×1 combiner in FIG. 1B. Further,Transceiver 2 disposed at the receiving end includes the waveguide-basedDLI as a 1×2 splitter (a deinterleaver) at least at the receivingportion.

In a specific embodiment, two low-frequency dither signals (tones) f1and f2 are added on respective two MZMs for controlling quadraturetransfer curve for modulating and locking the wavelength λ1 or λ2 oftransmitted light wave in corresponding CH1 or CH2 output. The tones f1and f2 are carried with the light wave signals through optical fiber andreceived by the Transceiver 2. The waveguide-based DLI in Transceiver 2is tuned to have a FSR equal to twice of the wavelength difference λ1−λ2to split the light wave into two interleaved parts and deliver a firstlight wave with λ1 and tone f1 to CH1 output and a second light wavewith λ2 and tone f2 to CH2 output. At the CH1 output a photodiode (PD)is used to detect (a taped portion of) the light wave signal withwavelength λ1 as well as to detect the tone f1. Similarly, another PD isalso used at CH2 to detect (a tapped portion of) both signal λ2 and tonef2. Tone f1 (f2) as detected by the PD is converted to an electricalsignal. The Transceiver 2 further includes a feedback circuit 150coupled between the PDs and the heater of DLI in the receiver portion ofTransceiver 2 and configured to use the converted electrical signalbased on f1 (f2) tone as a control signal to tune interference spectrumsof the light wave with two wavelengths passing through the DLI (seedescription for FIG. 2). As the result, the f1 (f2) component at the CH1(CH2) output is tuned be maximized so that the corresponding transmittedlight wave at CH1 (CH2) output can be locked to its pre-selectwavelength λ1 (λ2), e.g., at a specific wavelength of ITU grid withleast interference from the other wavelength λ2 (λ1). Alternatively, bymaintaining a stable power ratio between two electrical signals detectedby two PDs in CH1 and CH2, the figure of merit of the modulation andwavelength control of two-channel DEMUX/MUX can be obtained.

FIG. 4 is a simplified diagram of a pair of two-channel transceiversusing tapped DFB signal for wavelength controlled transmission accordingto another embodiment of the present invention. This diagram is merelyan example, which should not unduly limit the scope of the claims. Oneof ordinary skill in the art would recognize many variations,alternatives, and modifications. As shown, the pair of two-channeltransceivers (Transceiver 1 and Transceiver 2) is respectively disposedat transmitting and receiving ends as MUX and DEMUX of a two-way trafficoptical communication loop including wavelength control. In thetransmission path of the optical communication loop, Transceiver 1 isconfigured to use two DFB laser sources with light wavelength at λ1 andλ2 being respectively modulated on two MZMs to form two optical signalsbefore being combined into one output for transmitting through opticalfiber. In an embodiment, the DFB laser wavelengths λ1 and λ2 areselected to be two neighboring channels of ITU grid with a channelspacing of 100 GHz, or 50 GHz, or 25 GHz, or others. The Transceiver 1can be the two-channel transceiver 100 described in FIG. 1A.Alternatively, the Transceiver 1 can be the two-channel transceiver 100Bwith the waveguide-based DLI as a 2×1 combiner in FIG. 1B. Further,Transceiver 2 disposed at the receiving end includes the waveguide-basedDLI as a 1×2 splitter (a deinterleaver) at least at the receivingportion.

The DLI in receiver portion of each Transceiver needs to be tuned sothat λ1 goes to CH1 PD and λ2 goes to CH2 PD to maintain good wavelengthcontrol during optical signal transmission and receiving. In a specificembodiment, the wavelength control can be done by using the DFB laserwavelength (presumably been locked at certain ITU grid) as a wavelengthreference. Particularly, the Transceiver 1 includes a first power tapdevice 116 coupled to, e.g., CH2 (or CH1) input that receives light wavewith wavelength λ2 (or λ1), to draw a small portion (2-10%) of lightwave λ2 (or λ1) component from CH2 DFB (or CH1 DFB) laser source. Thisλ2 (or λ1) component is fed (in a reverse direction) via a second powertap device 126 into CH1 (CH2) output path of the DLI 120 in the receiverportion of the Transceiver 1. After this tapped λ2 (or λ1) componentpasses through the DLI 120 and is detected by a photodiode (PDr)disposed next to the common input of the DLI 120, this λ2 (or λ1)component as a wavelength reference is converted to an electricalsignal. The Transceiver 1 further includes a feedback circuit 160 tofeed this electrical signal back to the heater of the same DLI 120 ofthe Transceiver 1 for tuning corresponding interference spectrums of thelight wave (from return trip of the communication loop) with bothwavelengths λ1 and λ2 so that the heater on the DLI 120 is tuned. In anembodiment, the heater power of the DLI 120 is tuned to have the λ2 (orλ1) power at PDr is maximized. As the result, the DLI 120 is set to makethe light wave with locked λ1 goes to CH1 PD and the light wave withlocked λ2 goes to CH2 PD as desired. Here the light waves belong tosignals of the return trip of the communication loop.

In an alternative embodiment, the above wavelength control scheme usingpower tap from input DFB light component as wavelength reference can bealso implemented to tune DLI in the Transceiver 2 in substantiallysimilar fashion. As the result, the light wave in forward trip of thecommunication loop is also able to be well controlled to make the lightwave with locked λ1 goes to CH1 PD and the light wave with locked λ2goes to CH2 PD as desired.

FIG. 5 is a simplified diagram of a pair of two-channel transceiversusing tapped DFB signal after MZM including tones for wavelengthcontrolled transmission according to yet another embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown, the pair of two-channel transceivers (Transceiver 1 andTransceiver 2) is respectively disposed at transmitting and receivingends as MUX and DEMUX of a two-way traffic optical communication loopincluding wavelength control. In the transmission path of the opticalcommunication loop, Transceiver 1 is configured to use two DFB lasersources with light wavelength at λ1 and λ2 being respectively modulatedon two MZMs to form two optical signals before being combined into oneoutput for transmitting through optical fiber. In an embodiment, the DFBlaser wavelengths λ1 and λ2 are selected to be two neighboring channelsof ITU grid with a channel spacing of 100 GHz, or 50 GHz, or 25 GHz, orothers. The Transceiver 1 can be the two-channel transceiver 100described in FIG. 1A. Alternatively, the Transceiver 1 can be thetwo-channel transceiver 100B with the waveguide-based DLI as a 2×1combiner in FIG. 1B. Further, Transceiver 2 disposed at the receivingend includes the waveguide-based DLI as a 1×2 splitter (a deinterleaver)at least at the receiving portion.

In an embodiment, the tone (or dither) signal f1 or f2 is used tocontrol the heater on the receiver DLI 120 in Transceiver 1. A smallportion (typically 2˜10%) of CH2 (or CH1) signal λ2 (or λ1) with tone f2(or f1) is tapped via a first power tap device 117 disposed after theMZM and fed via a second power tap device 127 into the receiver DLI 120from CH1 PD paths (or CH2 PD path). After this tapped λ2 (or λ1)component passes through the DLI 120 and is detected by a photodiode(PDr) disposed next to the common input of the DLI 120, this λ2 (or λ1)component as a wavelength reference is converted to an electricalsignal. The Transceiver 1 further includes a feedback circuit 170 tofeed this electrical signal back to the heater of the same DLI 120 ofthe Transceiver 1 for tuning corresponding interference spectrums of thelight wave (from return trip of the communication loop) with bothwavelengths λ1 and λ2 so that the heater on the DLI 120 is tuned. In anembodiment, the heater power of the DLI 120 is tuned to have the λ2 (orλ1) power at PDr is maximized. Here the light waves belong to signals ofthe return trip of the communication loop. In another embodiment, theheater on the DLI 120 is tuned so that a second harmonic of the tonefrequency 2 f 2 (or 2 f 1) is maximized. As the result, the DLI 120 isset to make the light wave with locked λ1 goes to CH1 PD and the lightwave with locked λ2 goes to CH2 PD as desired.

In an alternative embodiment, the above wavelength control scheme usingpower tap from input DFB light component as wavelength reference can bealso implemented to tune DLI in the Transceiver 2 in substantiallysimilar fashion. As the result, the light wave in forward trip of thecommunication loop is also able to be well controlled to make the lightwave with locked λ1 goes to CH1 PD and the light wave with locked λ2goes to CH2 PD as desired.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A silicon photonic transceiver system comprising: a first optical input port for receiving a first optical signal; a second optical input port for receiving a second optical signal; a first modulator coupled to the first optical input port and being configured to modulate the first optical signal at a first wavelength; a second modulator coupled the second optical input port and being configured to modulate the second optical signal at a second wavelength; a combiner being configured to generate a transmission signal using the modulated first optical signal and the modulated second optical signal; a third optical input port for receiving a third optical signal; and a first delay-line-interferometer (DLI) being configured to split the third optical signal into a fourth optical signal and a fifth optical signal, the fourth optical signal being characterized by the first wavelength, the fifth optical signal being characterized by the second wavelength.
 2. The system of claim 1 wherein the first modulator comprises a Mach-Zehnder modulator.
 3. The system of claim 1 wherein the combiner comprises a 2×1 multimode-interference power combiner or a 2×1 3 dB power combiner.
 4. The system of claim 1 wherein the combiner comprises a second DLI.
 5. The system of claim 1 wherein the transmission signal comprises a dither signal.
 6. The system of claim 1 wherein the first DLI comprises a resistive heater for providing wavelength control.
 7. The system of claim 1 wherein the first DLI is waveguide based.
 8. The system of claim 1 further comprising a photodetector for processing the fourth optical signal.
 9. The system of claim 1 wherein the first DLI comprises a silicon-based waveguide device having two optical paths with one path being made longer than another optical path by a predetermined length and being formed on a silicon-on-insulator substrate and comprises a resistive heater being built around the one optical path having the longer length, the longer length corresponding a delayed phase leading to a free-spectral range of an interference spectrum of the third/fourth optical signal, the free-spectral range being equal to twice of channel spacing between the first wavelength and the second wavelength.
 10. The system of claim 1 wherein the first optical signal comprises a passband characterized by a substantially symmetric curve relative to the first and second wavelength.
 11. The system of claim 1 wherein the first modulator is configured to add a dither signal to the modulated first optical signal.
 12. The system of claim 1 wherein the first DLI comprises a feedback circuit and a resistive heater to tune an interference spectrum of the fourth optical signal and the fifth optical signal.
 13. The system of claim 1 further comprising a reference photodiode coupled to the first DLI.
 14. A method for locking channel wavelengths, the method comprising: receiving a first input signal and a second input signal; generating a first optical signal by modulating the first input signal and inserting a first dither signal, the first optical signal is characterized by a first wavelength; generating a second optical signal by modulating the second input signal and inserting a second dither signal, the second optical signal is characterized by a second wavelength; obtaining a transmission signal by combining the first optical signal and the second optical signal; transmitting the transmission signal through an optical link; receiving the transmission signal; splitting the transmission signal into a third optical signal and a fourth optical signal using a delay-line interferometer (DLI), the DLI being characterized by a free-spectral range equal to twice of channel spacing between the first wavelength and the second wavelength; converting the third optical signal to a first electrical signal; and converting the fourth optical signal to a second electrical signal.
 15. The method of claim 14 wherein the transmission signal comprises a fifth optical signal.
 16. The method of claim 14 further comprising converting the first input signal into the first optical signal using a DFB laser.
 17. The method of claim 14 further comprising tuning a heater of the DLI to maximize the first electrical signal and the second electrical signal.
 18. The method of claim 14 wherein the third optical signal and the fourth optical signal are substantially free from element of the first wavelength and the second wavelength.
 19. An optical transceiver system comprising: a first modulator configured to modulate a first optical signal at a first wavelength; a second modulator being configured to modulate a second optical signal at a second wavelength; a combiner being configured to generate a transmission signal using the modulated first optical signal and the modulated second optical signal; a third optical input port for receiving a third input signal; and a delay-line-interferometer (DLI) being configured to split the third input signal into a fourth optical signal and the fifth optical signal, the fourth optical signal being characterized by the first wavelength, the fifth optical signal being characterized by the second wavelength a first photodiode for converting the fourth optical signal into a first electrical signal; and a second photodiode for converting the fifth optical signal into a second electrical signal.
 20. The method of claim 19 further comprising a reference photodiode coupled to the DLI. 